In a wireless multiple access communication system, the wireless traffic channel resource, e.g., bandwidth and time interval, is shared by all the wireless terminals, i.e., mobile units, in a particular cell. Efficient allocation of this traffic channel resource is very important, as it directly impacts the utilization of the traffic channel resource and the quality of service perceived by individual wireless terminal users. One such wireless communications system is the Orthogonal Frequency Division Multiplexing (OFDM) based Multiple Access system.
OFDM represents a different system design approach. It can be considered a combination of modulation and multiple access schemes that segment a communications channel in such a way that many users can share it. Whereas TDMA segments according to time and CDMA segments according to spreading codes, OFDM segments according to frequency. It is a technique that divides the spectrum into a number of equally spaced tones, and carries a portion of a user's information on each tone. OFDM can be viewed as one form of frequency division multiplexing (FDM). However, OFDM has an important special property, in that each tone is orthogonal with every other tone. FDM typically requires frequency guard bands between the frequencies, so that they do not interfere with each other. OFDM allows the spectrum of each tone to overlap, and since they are orthogonal, they do not interfere with each other. By allowing the tones to overlap, the overall amount of spectrum occupied is reduced.
OFDM can also be considered a multiple access technique, since an individual tone or groups of tones can be assigned to different users. Multiple users share a given bandwidth in this manner, yielding orthogonal frequency division multiple access, or OFDMA. Each user may be assigned a predetermined number of tones when they have information to send, or alternatively, a user can be assigned a variable number of tones based on the amount of information they have to send. Assignments are controlled by the media access control (MAC) layer, which schedules the resource assignments based on user demand.
In a wideband wireless communications system, signal may decrease from frequency selective fading, due to multi-path transmissions. Conventional OFDM systems have proposed overcoming frequency selective fading by dividing total bandwidth into a plurality of subcarriers, such that the bandwidth on each subcarrier is sufficiently narrow to enable the data modulation symbols carried by that subcarrier to experience relatively flat fading.
OFDMA systems commonly use an OFDM modulation technique to multiplex the data traffic of a plurality of mobile stations, in both frequency and time. In a cellular communication network based on OFDMA, a base station communicates with mobile stations that are within its coverage area by using signals that are orthogonal in frequency, thereby eliminating intra-cell interference.
For a multi-carrier (OFDM) system, the transmission resource is the frequency-time block. To support hybrid automatic request (HARQ)—and advanced re-transmission strategy that allows for retransmissions directly at the physical/MAC layer, without involving higher layer mechanisms and inducing delay—the time line may be divided into several intervals, and the transmission of one packet may occupy only one interval. In addition, frequency allocation normally consists of a group of subcarriers.
Utilizing the concept of a channel tree, transmission granularity may be considered as a base node. Transmission granularity refers to the size of objects transmitted and received as a unit. For example, packet networks send and receive data in packets. Even if only some of the bits of a packet are erased or corrupted, the whole packet is discarded and mechanisms (e.g., forward error correction, request for resend) are activated to recover the packet as a whole.
Thus, such objects are either received error-free or are erased in their entirety. In some applications, an object's size could be the size of the transmission packets or could be smaller. In a channel tree, a parent node may have a set of child nodes. The relationships between the children and parent nodes do not overlap. It is understood that much larger resource allocation is possible with a higher layer tree node.
To reduce assignment signaling overhead, a system may use “synchronous HARQ” and provide support for “sticky” assignments. With synchronous HARQ, resources for successive retransmissions are not independently scheduled, but rather are retained for all re-transmissions associated with a packet. Thus, assignment of a set of hop-ports applies to an interval (or “interlace”). Assignments on different interlaces are independent, and an AT may be given resources on multiple interlaces.
Assignments can be sticky or non-sticky. Sticky assignments are useful to reduce assignment overhead required when it is beneficial to schedule multiple users simultaneously, and to eliminate request latency for RL transmissions. When an assignment is non-sticky, the assignment expires on successful packet decode, or when the packet fails to decode after the maximum number of H-ARQ retransmissions allowed for the packet. When assignments are sticky, the assignment persists as long as the assigned resource is in use. An assignment is in use as long as either a packet or an erasure sequence is transmitted using the assignment. The erasure sequence is simply a one-bit “keep alive” indication used to inform the receiver that the assignment should be retained even though a data packet might not be available for transmission using the assignment. If neither a packet nor an erasure sequence is transmitted using the assignment, the assignment expires and the resources are free for subsequent allocation. In addition, it is possible for the AP to send an explicit message that ends an assignment.
To reduce overhead required to specify sets of hop ports in a system, a finite space of channel IDs are defined that map to specific sets of hop ports, and are used to communicate assignments to ATs. Because assignments can be sticky, and to combat fragmentation of resources in the system due to the finite mapping of channel IDs, the system supports supplemental assignments that add sets of hop ports to the existing set allocated to an AT for an interlace. Such supplemental assignments are sent to augment an AT's allocation between packet transmissions.
The mapping between channel IDs and hop-ports is defined using the channel tree (as mentioned above), such as the one illustrated in FIG. 1. Each node on the tree is given a unique channel ID. For example, in FIG. 1, the channel tree shows that there are 32 base nodes in the system, namely L3164, L3264, . . . , and L6264, wherein the superscript denotes a specific node layer and the subscript denotes a specific node ID. As a further example one can see that node L1532 consists of base node L3164 and L3264 and thus can be used to transmit larger traffic. Further, each base node (nodes at the bottom of the tree) is mapped to a set of hop ports. A channel ID then maps to the set of hop ports mapped by the base nodes under the node of the channel ID.
Generally speaking, there are two kinds of resource assignments: sticky and non sticky. In order to make the communication between base stations and terminals more efficient, a concept of sticky assignments is illustrated. Sticky assignments are useful in a scheduled data transmission system in cases where many users are competing for limited assignment message resources. A sticky assignment is when a resource (e.g., a channel) that is assigned to a particular user continues to be available to that user after the standard unit of transmission (e.g., packet) is completed. Thus, a new assignment message is not necessary to enable that user to continue transmission.
When an assignment is sticky, the assignment persists as long as the assigned resource is in use. In terms of reducing signaling overhead, sticky assignments are useful for the long standing and non-bursty traffic, such as VOIP. However, it is likely that the resource assignment granularity is larger than one traffic loading. In this case, it is helpful to share the same resource with several users or traffics. When an assignment is non-sticky, the assignment expires on the end this packet transmission.
Thus, significant signaling overhead is present in OFDMA systems. Receiving terminals need to know which sub-carriers are assigned to them; and this affects the system's performance. Sub-carrier attenuations may be correlated over time, so few assignments change from one down-link phase to the next.
Therefore, there is a need for structures and methods that allow multiple users to share air interface resources, including physical layer packet formats and signaling methods that optimize overall system performance.